The application of siRNA to medical treatments is increasingly expected because siRNA can knock down target mRNA specifically and effectively. The development of an effective delivery system is indispensable to applying siRNA to medical treatments. In recent years, it has been clarified in clinical trials that the therapeutic effect on age-related macular degeneration (CNV) by intraocular administration of naked siRNA does not result from a sequence-specific gene knockdown effect mediated by siRNA, but rather results from a non-sequence-specific effect via recognition by the cell surface Toll-like receptor-3 (TLR-3); thus, the development of a carrier, which is stable outside of cells and is capable of accurately delivering siRNA into the cells in any in vivo application of siRNA, is considered to be important.
Cationic polymers have been provided as carriers that introduce nucleic acid into eukaryotic cells by forming a complex with a small-molecule nucleic acid such as siRNA under physiological conditions and cause the nucleic acid to be expressed (for example, Patent Literature 1). In case nucleic acid such as siRNA will be applied to medical treatments, the nucleic acid preferably has a high blood retentivity from the viewpoint of sustaining the effect, and there is still room for improvement in the blood retention capabilities of conventional cationic polymers.